Are Reservoirs A Waste?

September 12, 2008

A lot of water debates in the western United States these days boil down to the question of whether
desert cities or desert farms are a worse idea. On the one hand, growing water-intensive crops like alfalfa
in places where it doesn't rain seems to make little sense. On
the other hand, cities (and their suburbs) use a lot of water for lawns, golf
courses, and swimming pools, none of which is exactly necessary to sustain
life. The problem, of course, is that there are people who stand to lose their
livelihoods if either of these uses gets cut.

Still, farmers and
suburbanites should agree on at least one thing: Desert reservoirs are a
waste. Evaporation rates are a function of temperature and lack of humidity, and
the deserts of the West are nothing if not hot and dry. Lake Powell and Lake
Mead, the two biggest reservoirs on the Colorado, lose an average
of 1.4 million acre-feet each year to evaporation. That's almost one-tenth of
the Colorado River's annual flow and more than twice the annual water
use of Los Angeles.

It'd be a much better idea to store that water
underground. Naturally-existing aquifers provide a logical place to do this, and
there are several proven ways—involving injection wells and porous-bottomed
ponds—to put water back into them. And, once the water's underground, very little
of it escapes. The managers of the Kern County water bank, one of the largest in
the country, calculate that they lose just 2 to 5 percent of the water they put into their
system—mostly as the result of evaporation while the water is sitting in ponds
soaking into the ground. The percentage lost is essentially the same whether the
water stays in storage for two years or ten. By comparison, Lake Mead and Lake
Powell lose around 5 percent of their water each year. If you're
using them to smooth multiyear wet/dry cycles, that adds up fast.

As climate change makes rainfall patterns more variable, dry regions
around the world are going to find themselves needing to store water for years
at a time. The majority of that storage should be happening in underground
aquifers rather than aboveground lakes. It won't make water skiers very happy,
but for everyone else there will be a whole lot more water to go around.